Solutions comprising iron can be found in many industrial applications. For instance, steel pickling facilities use acid, e.g. hydrochloric acid or sulphuric acid, to treat surfaces of steel objects, mostly steel plates. The acid is provided in aqueous solution and the waste resulting from the pickling is called waste pickling liquid or waste pickle liquor (WPL), which typically is an aqueous solution comprising iron, mainly ferrous iron, e.g. in the form of a ferrous chloride or ferrous sulphate solution. The iron content in WPL is typically less than 10 wt %, usually between 6 and 9 wt % of iron, mainly in the form of ferrous chloride (FeCl2). Iron-comprising solutions are further used in water treatment, in particular ferric chloride solutions which can be used as a coagulating and/or flocculating agent. The iron content of such ferric chloride solutions is typically of the order of 15 wt %. However, higher iron content solutions can be preferred, in particular for transporting the ferric chloride solution to a water purification facility or to an intermediate distribution point to reduce transport costs or to store the solution in a storage tank. The iron content of the solution can then be reduced to the desired concentration in situ or at the distribution point by dilution or the solution of high ferric chloride concentration can be used as such for e.g. water purification.
Document WO 2012/075109 discloses a method for making a high iron-content ferric chloride solution, comprising evaporating water from a ferric chloride solution having an iron content of 15 wt % or less to increase the iron content to 23-26 wt %. The document also discloses a process for reconstituting an aqueous ferric chloride solution having an iron content of about 23 to 26 weight percent by diluting a ferric chloride solution having an iron content of about 23 to 26 weight percent with water to a lower iron content. Document WO 2012/075109 further discloses the exemplary embodiment of either mixing solid iron containing materials with hydrochloric acid to obtain a ferric chloride solution with an iron content of about 15 wt %, or of oxidizing spent pickling liquors to obtain a ferric chloride solution with an iron content of about 15 wt %. This ferric chloride solution with an iron content of about 15 wt % can then be evaporated to further increase the iron content to about 23 to about 26 wt %.
The problem with the prior art technique is that it requires intensive heating to evaporate the water from the ferric chloride solution.
There remains a need in the art for an improved process for making a high iron-content solution, preferably comprising ferric chloride, which overcomes the problem mentioned above. There also remains a need in the art for improved methods or devices for storing, transporting and/or moving such as solution, as well as for diluting such a solution.
The invention thereto aims to provide a method for making a high iron-content solution comprising ferric chloride at an increased concentration, which is more energy efficient than the prior art method.